Field of the Invention
The present invention relates to a packaging bag for microwave ovens, in which food is housed and heated by a microwave oven.
Description of Related Art
As a packaging bag hermetically filled with food such as retort food or frozen food is heated by a microwave oven, the food is warmed or cooked. In this case, an internal pressure of the packaging bag is raised by water vapor or the like generated from the food along with the heating, and the packaging bag may burst. Accordingly, before the packaging bag is heated by a microwave oven, by, for example, partly opening the packaging bag in advance or by forming a hole in a packaging bag body as a countermeasure to prevent the bursting of the packaging bag, the water vapor or the like generated within the packaging bag along with the heating are usually discharged to the outside of the packaging bag. However, in this method, since the water vapor generated by the heating in the microwave oven is directly discharged to the outside of the packaging bag, heating and steaming effects caused by the water vapor are reduced, and a taste of the food is reduced.
Accordingly, for example, in Japanese Unexamined Patent Application, First Publication No. 2002-249176 (hereinafter referred to as Patent Document 1), a packaging bag is proposed in which water vapor generated by heating in a microwave oven is not directly discharged to the outside of the packaging bag and the water vapor can be automatically discharged to the outside after an internal pressure is raised to a certain extent. In the packaging bag described in Patent Document 1, a sealed circular vapor release part at which a pair of films facing each other are sealed is provided at a corner portion, and a through-hole is formed in the center of the sealed vapor release part. If the internal pressure of the packaging bag is raised by heating the packaging bag in the microwave oven, the sealed circular vapor release part is peeled. As a result, the inside and outside of the packaging bag communicate with each other through the through-hole, and vapor inside the packaging bag is discharged to the outside.
However, in the packaging bag described in Patent Document 1, when the through-hole is provided in the sealed vapor release part, a method of punching the sealed vapor release part with a punching device is mainly adopted. In this case, generation of an excised piece (a released gas) cannot be avoided. For this reason, in producing the packaging bag described in Patent Document 1, troublesome work of collecting all the excised pieces is required. If a leakage of the collection or the like occurs, there is a possibility of the excised piece being mixed into a product.
The present invention was conceived in view of the above-described circumstances, and an object of the present invention is to provide a packaging bag for microwave ovens in which, when an internal pressure of the packaging bag is raised by heating in a microwave oven, the inside and outside of the packaging bag can automatically communicate with each other to release the internal pressure to the outside, and an excised piece can be prevented from occurring in a process of producing the packaging bag.